hi i got the same error message today too (all i did was add all of the omni mods from the in-game mod searching feature) (i clicked the button to disable omnicrystal)
then it mentioned another error so i clicked to disable omni fluid after this message:
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7.794 Script @omnimatter_wood/prototypes/override.lua:3: omniwood: dry-hairy-tree
7.794 Script @omnimatter_wood/prototypes/override.lua:3: omniwood: dead-dry-hairy-tree
7.894 Loading mod pyindustry 1.0.1 (data-final-fixes.lua)
8.004 Loading mod omnimatter_science 0.3.4 (data-final-fixes.lua)
8.087 Script @omnimatter_science/data-final-fixes.lua:257: Something didn't go as I wanted and this many remains: 3 and they are
8.087 Script @omnimatter_science/data-final-fixes.lua:259: vtk-deepcore-mining
8.087 Script @omnimatter_science/data-final-fixes.lua:259: deep-mine
8.087 Script @omnimatter_science/data-final-fixes.lua:259: deep-mine-2
8.179 Loading mod reverse-factory 4.0.13 (data-final-fixes.lua)
8.842 Loading mod omnimatter_fluid 0.1.1 (data-final-fixes.lua)
8.848 Script @omnilib/prototypes/recipe-standard.lua:20: standardizing: offshore-pump
8.849 Script @omnilib/prototypes/recipe-standard.lua:20: standardizing: boiler
8.849 Script @omnilib/prototypes/recipe-standard.lua:20: standardizing: heat-exchanger
8.849 Script @omnilib/prototypes/recipe-standard.lua:20: standardizing: bio_boiler
8.849 Script @omnilib/prototypes/recipe-standard.lua:20: standardizing: n-cryamond-boiler
8.850 Error ModManager.cpp:957: Failed to load mod "omnimatter_fluid": omnimatter_fluid/data-final-fixes.lua:252: attempt to perform arithmetic on a nil value
8.851 Loading mod core 0.0.0 (data.lua)
8.935 Checksum for core: 1506903607
9.117 Error ModManager.cpp:957: Error in assignID, recipe-category with name 'crafting' does not exist.
Source: starting-area (noise-layer).
9.303 Loaded shader file C:/Games/Factorio_x64_0.15.31/Factorio_0.15.31/data/core/graphics/shaders/game.cso
9.303 Loaded shader file C:/Games/Factorio_x64_0.15.31/Factorio_0.15.31/data/core/graphics/shaders/zoom-to-world.cso
9.309 Initial atlas bitmap size is 16384
9.309 Created atlas bitmap 192x5
9.310 Created atlas bitmap 132x24
9.398 Sprites loaded
9.398 Convert atlas 132x24 to: trilinear-filtering
9.401 Custom inputs active: 0
9.420 Factorio initialised
9.420 Mods to disable:Failed to load mods: omnimatter_fluid/data-final-fixes.lua:252: attempt to perform arithmetic on a nil value
Mods to be disabled:
• omnimatter_fluid
235.119 Info AppManager.cpp:769: Restarting Factorio
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